The grieving mother of an 8-month-old boy killed by a dog bite last week called the death a “tragic accident” and expressed disbelief that the pit bull that seemed to watch over the infant like a protective parent would ultimately end the baby’s life.

“Never in my life I thought that would happen,” the single mother said Monday from her Lemon Grove apartment. She agreed to be interviewed on the condition that her name not be used.

Sheriff’s detectives are continuing to investigate the death of Tyzhel McWilliams. No arrests have been made in the case.

The investigation will be turned over to the District Attorney’s Office, which will review the case and determine if charges will be filed, sheriff’s homicide Lt. Larry Nesbit said Tuesday.

The 27-year-old woman moved into the single-story apartment on West Street two years ago with a girlfriend who had three male pit bulls — Blue and his two pups, Zulu and Hercules.

The move was a fresh start from the woman’s past of homelessness and abuse. Her epilepsy made the pregnancy high-risk, but she was determined to carry the child, she said.

When Tyzhel was born, the mother said she had no qualms about letting the dogs interact with her son.

“They loved him from the beginning,” she said.

On Thursday evening, Tyzhel was playing in the living room while his mother and the roommate talked in a bedroom across the hall. Three-year-old Zulu was in the room with them. The bedroom has a partial view into the living room.

When the baby crawled into the bedroom, Zulu bit him on the head once and let go, the mother said.

“It just happened so quickly,” she said.

Authorities who responded to the 911 call found the wounded child cradled in his mother’s arms. He died at a hospital.

“He was the light of my world. I’d do anything for him,” she said. “He was the best thing in my life.”

The mother, who is unemployed, said she has no contact with Tyzhel’s father.

The mother said the other dogs were not involved in the death, which investigators confirmed. One was asleep in a crate in another bedroom and the other was in the fenced backyard, she said.

The three dogs were impounded at the Chula Vista Animal Care Facility.

Zulu was euthanized Tuesday, and the two other dogs have been released back to the owner because they had no reported history of violence, a city of Chula Vista spokeswoman said.

Both women described the dogs as friendly and well-behaved. They said the canines had killed two cats since living there; in both instances, the cats had entered the fenced backyard.

As the mother prepares to lay her only child to rest this week, she grapples with the question: What would she have done differently?

“I guess not have dogs around the baby,” she said, “but I really had no reason to think to treat the dog and baby differently.”

She advised other pet owners to be even more cautious than they think they need to be when bringing dogs — whether big or small — around small children.

“You never know,” she said.

Nationally, children ages 5 to 9 are bitten at the highest rate, according to the Centers for Disease Control and Prevention. Almost two-thirds of injuries to children 4 and under were to the head and neck region.

Of the 2,746 dog bites reported to the county Department of Animal Services since July 1, 425 were to children 12 and under. The majority of dog bites, 42 percent, were to adults ages 20 to 55. The agency serves all unincorporated areas of the county, as well as San Diego, Santee, Carlsbad, Del Mar, Solana Beach and Encinitas.

Madeline Gabriel, who teaches pet safety classes to new parents at San Diego hospitals, said people need to change their thinking about how dogs and young children interact.

“We want the fantasy of the dog loves the baby ... but it blinds us to the inappropriateness of it,” Gabriel said.

“The baby can be with you and the dog can be with you,” she said. “But when you look at the dog and baby together, you have to realize ‘I’ve just chosen to put the safety and well-being of my family on the dog and the baby.’”

Gabriel teaches parent to act not just as a supervisor, but as a guide when the dog and baby are in the same space. Parents should physically stay between the baby and dog, guiding the baby’s behavior while at the same time reassuring the dog.

“An under-4 child is not consistently going to choose behavior the dog likes anyway,” she added.

For more information on the “Dogs and Babies — Play it Safe” classes at Sharp Mary Birch Hospital and Scripps Well Being at UTC, go to dogsandbabieslearning.com.

Donations to the family can be made out to Baby Tyzhel’s Donation Account at the U.S. Bank at 5100 Federal Blvd., San Diego, CA, 92105.